“The Dark Side,” Jane Mayer’s gripping new account of the war on terror, is really the story of two wars: the far-flung battle against Islamic radicalism, and the bitter, closed-doors domestic struggle over whether the president should have limitless power to wage it. The euphemistically named but often grisly particulars of the fight against Al Qaeda — the “extraordinary renditions” by hooded agents in unmarked planes, the secret “black site” prisons across the globe, the “enhanced” interrogation techniques, the “reverse rendition” of detainees lucky enough to be found innocent and dumped blindfolded at remote borders — are harrowingly recounted here, complete with fresh revelations. But in Ms. Mayer’s hands the story of bureaucratic jockeying in well-upholstered offices and in the fine print of legal documents makes for an equally absorbing and disturbing story. It’s a cage match between the Constitution and a cabal of ideological extremists, and the Constitution goes down.